1. Field of the Invention
The present invention relates to a semiconductor memory device and a method of manufacturing the same. More particularly, the present invention relates to a magnetic random access memory having a magnetic tunneling junction (MTJ) layer including a tunneling film of uniform thickness and a method of manufacturing the same.
2. Description of the Related Art
A magnetic random access memory (MRAM) is a memory device used to read and write data utilizing a property of resistance variation of a magnetic tunneling junction (MTJ) cell depending on a direction of magnetization of the MTJ cell. The MTJ cell includes a tunneling film and magnetic films on and under an insulating film.
MRAM is as highly integrated as a dynamic random access memory (DRAM) and capable of operating at high speed like a static random access memory (SRAM). In addition, MRAM can maintain data for a relatively long time without a refreshing process, similar to a flash memory device. Accordingly, MRAM has the advantages of both non-volatile and volatile memory devices.
Conventionally, an MRAM includes a transistor T and an MTJ layer electrically connected to the transistor. The transistor T performs a switching role, and data, i.e., “0” and “1”, are recorded in the MTJ layer.
More specifically, referring to FIG. 1, which illustrates a cross-sectional view of a conventional MRAM, a conventional MRAM has a gate stack 12 containing a gate electrode on a substrate 10. Source and drain regions 14 and 16 are formed in the substrate 10 on opposite sides of the gate stack 12. The gate stack 12 and the source and the drain regions 14 and 16 formed on the substrate 10 constitute a transistor T. The transistor T is separated from adjacent transistors (not shown) by field oxide films 11 formed on the substrate 10. An interlayer insulating layer 18 covering the transistor T and the field oxide films 11 is formed on the substrate 10. A data line 20 is formed above the gate stack 12 in the interlayer insulating layer 18. The data line 20 is separated from and parallel to the gate stack 12. A contact hole 22 exposing the source region 14 is formed in the interlayer insulating layer 18. The contact hole 22 is filled with a conductive plug 24. A pad conductive layer 26 is formed on the interlayer insulating layer 18 in contact with a top surface of the conductive plug 24. The pad conductive layer 26 is formed above the data line 20. An MTJ layer S is formed on the pad conductive layer 26 directly above the data line 20. A second interlayer insulating layer 28 is formed covering the MTJ layer S and the pad conductive layer 26. A via hole 30 is formed in the second interlayer insulating layer 28 to expose the MTJ layer S. A bit line 32 is formed on the second interlayer insulating layer 28 perpendicular to the gate stack 12 and the data line 20. The bit line 32 is also formed in the via hole 30 to contact the MTJ layer S.
FIG. 2 illustrates a cross-sectional view of the conventional MTJ layer of the conventional MRAM depicted in FIG. 1. Referring to FIG. 2, the MTJ layer S of the conventional MRAM includes lower magnetic films S1, S2, and S3, a tunneling film S4, and upper magnetic films S5 and S6, stacked sequentially on the pad conductive layer 26. The lower magnetic films S1, S2, and S3 are a lower electrode S1, a pinning magnetic film S2, and a pinned magnetic film S3. The tunneling film S4 is an aluminum oxide film. The aluminum oxide film is formed by oxidation of an aluminum film formed on the pinned magnetic film S3 using one of a plasma oxidation method, a UV oxidation method, a natural oxidation method, and an ozone oxidation method. The upper magnetic films S5 and S6 are a free ferromagnetic film S5 and a capping film S6.
The conventional MRAM described above suffers from the following disadvantages. First, as depicted in FIG. 2, a grain boundary 34 that appears when forming the lower electrode S1 tends to extend to the tunneling film S4 through the pinning magnetic film S2 and the pinned magnetic film S3. If the grain boundary 34 extends through the pinned magnetic film S3, oxygen diffuses along the grain boundary 34 into the pinned magnetic film S3 and oxidizes an adjacent area of the pinned magnetic film S3 during the oxidation process for forming the tunneling film S4. This results in a difference in a thickness of the tunneling film S4 between the area contacting the grain boundary 34 and areas not contacting the grain boundary 34.
FIG. 3 is a tunneling electron microscope (TEM) image of the conventional MTJ layer depicted in FIG. 2 and shows this difference in thickness. In the MTJ layer shown in FIG. 3, the lower electrode S1 (not shown) is formed of Ta/Ru, the pinning magnetic film S2 (not shown) is formed of IrMn, and the pinned magnetic film S3 is formed of synthetic artificial ferromagnetic (SAF). The tunneling film S4 is an aluminum oxide film having a thickness of 15 Å, and the free ferromagnetic film S5 is formed of CoFe. As shown in FIG. 3, a thickness t2 of the tunneling film S4 where it contacts the grain boundary 34 is thicker than a thickness t1 of the tunneling film S4 where it does not contact the grain boundary 34.
If the thickness of the tunneling film S4 is not uniform, a thin area may cause a weak point. When a weak point exists in the tunneling film S4, current may concentrate on that weak point, which drastically reduces an insulation breakdown voltage of the tunneling film S4. Also, a switching uniformity decreases, and a cell resistance and a magnetoresistance (MR) could be reduced. Moreover, because the distribution of the grain boundary 34 generally differs from cell to cell, the distribution of the weak point also differs from cell to cell. Thus, each cell may have a different resistance. Therefore, deviation in resistance and MR between cells increases.
A second drawback of the conventional MRAM is that the tunneling film S4 is not flat. FIG. 4 is a schematic showing Neel coupling caused by waves, i.e., an uneven upper surface, in the tunneling oxide film S4 of the conventional MTJ layer depicted in FIG. 2. The tunneling film S4 depicted in FIG. 2 is drawn flat for simplicity, but, practically, the tunneling film S4 is not flat, as depicted in FIG. 4. Actually, the lower electrode S1 is formed with an uneven, i.e., wavy, upper surface. This wavy upper surface of the lower electrode S1 is transferred to the pinning magnetic film S2, the pinned magnetic film S3, and finally the tunneling film S4. Accordingly, the tunneling film S4 has the same wavy shape as the lower electrode S1. When the tunneling film S4 has a wavy surface, a switching field for switching the ferromagnetic film S5 is shifted widely by a Neel coupling between the pinned magnetic film S3 and the free ferromagnetic film S5. This problem may cause an error in the recording or reading of data.
FIG. 4 shows a problem caused by Neel coupling when waves having an amplitude of h and a frequency of λ are present on the tunneling film S4. In FIG. 4, tF represents a thickness of the free ferromagnetic film S5 stacked on the tunneling film S4, and tS represents a thickness of the tunneling film S4. HM represents a magnetic field for switching the free ferromagnetic film S5 (switching magnetic field). HN represents a magnetic field generated by Neel coupling due to the waves of the tunneling film S4 that causes a shift in the switching magnetic field (shift magnetic field). A degree of the shift in the switching magnetic field HM varies according to the magnitude of the shift magnetic field HN.
The shift magnetic field HN due to Neel coupling can be expressed as the following formula:
                              H          N                =                                                            π                2                            ⁢                              h                2                                                                    2                            ⁢              λ              ⁢                                                          ⁢                              t                F                                              ⁢                      M            P                    ⁢                      exp            ⁡                          (                                                -                  2                                ⁢                π                ⁢                                  2                                ⁢                                                      t                    S                                    /                  λ                                            )                                                          (        1        )            
A coupling energy density JN due to the presence of the shift magnetic field HN can be expressed as the following formula:
                              J          N                =                                                            π                2                            ⁢                              h                2                                                                    2                            ⁢              λ                                ⁢                      M            F                    ⁢                      M            P                    ⁢                      exp            ⁡                          (                                                -                  2                                ⁢                π                ⁢                                  2                                ⁢                                                      t                    S                                    /                  λ                                            )                                                          (        2        )            
Referring to mathematical formulas 1 and 2, the shorter the wavelength λ of the tunneling film S4, the larger the shift magnetic field HN and the coupling energy density JN. Conversely, the longer the wavelength λ, the smaller the shift magnetic field HN and the coupling energy density JN.
The shift magnetic field HN in the free ferromagnetic film S5 due to the unevenness of the tunneling film S4 in the conventional MTJ layer increases a coercivity of the free ferromagnetic film S5, and shifts the magnetic field for switching the free ferromagnetic film S5 by as much as the magnitude of the shift magnetic field HN. This results in errors in recording or reading data of the MTJ layer, thereby diminishing the reliability of the conventional MRAM device.